1. Field of the Invention
The present invention relates to a technique for removing metals contained in a quartz product by baking the quartz product which is a component of a heat treatment apparatus for heating semiconductor substrates.
2. Background Art
As one example of the heat treatment apparatus used in a semiconductor manufacturing process, there is known a vertical-type heat treatment apparatus which is a batch-type heat treatment apparatus. This vertical-type heat treatment apparatus comprises a heating furnace including a vertical-type reaction vessel which opens downwardly and a heater provided to surround the outside of the reaction vessel. Multiple sheets of semiconductor wafers (hereinafter referred to as wafers) are held in a shelf-like state by a wafer holding tool called wafer boat and carried into the reaction vessel from below, so as to perform a film forming process or the like, by oxidation, diffusion process or CVD, in the reaction vessel.
The reaction vessel, the wafer boat and a thermal insulation unit (or heat insulation unit) which constitute the heat treatment apparatus are usually formed from quartz. While such quartz products are machined through various steps starting from a quartz ingot, metallic contamination due to copper or the similar metal tends to occur because of contact with machining tools or effect from the working atmosphere. In particular, for the wafer boat, supporting portions for supporting wafers are composed of grooves or nails and require fine machining works for their formation, as such increasing the possibility of copper contamination. Quartz products are washed with hydrofluoric acid by a quartz product maker in order to remove copper present in the surface portion. However, the copper ions eluted into the hydrofluoric acid may tend to be coupled again to uncoupled sites of silicon atoms, resulting in leaving a trace amount of copper in the surface portion of the quartz products.
The quartz products are carried in a semiconductor manufacturing apparatus maker, assembled together, and shipped as a vertical-type heat treatment apparatus to a user. In this case, if the surface portion of each quartz product is contaminated with even a trace amount of copper, when the user starts the operation of the apparatus to carry out a heat treatment of wafers, the copper is heated up so as to make the molecular movement vigorous, as such a part of the copper is scattered into the heating atmosphere, then attaches to wafers and causes contamination of the wafers. In particular, for the wafer boat, since each wafer is placed thereon so as to directly contact with it, metals contained in the surface portion of the wafer boat are likely to be transferred into the wafer. In recent years, attempts to make semiconductor devices thinner and finer have been advanced. Accordingly, even though the wafer is contaminated by a trace amount of copper, electric characteristics of semiconductor devices may be badly affected, thus lowering the yield.
Patent Document 1 describes a baking process for quartz products at 1000° C. for 2 hours in the presence of hydrogen chloride gas and oxygen gas after assembling quartz products into a heat treatment apparatus and before effecting oxidation. The Patent Document 1 also discloses to conduct the baking before the incorporation of quartz products into a heat treatment apparatus, and describes that the number of copper atoms present in the surface portion of the quartz products can be reduced by carrying out such baking.
However, through various experiments, the inventor has found that, in some cases, the copper is not only attached to the surfaces of the quartz products, but, in fact, penetrates into the interior of each quartz product, and the copper concentration of the interior is higher than that of the surface portion in terms of the copper concentration profile in the depth direction. Therefore, the assessment for the copper a contamination of quartz products based only on the number of copper atoms in the surface portion of the quartz products can not successfully meet the prevention of wafer contamination.    Patent Document 1: TOKUKAI No. 2002-313787 (claim 1, Paragraphs [0017] to [0027])